124 THE THEORY OF THE GENE 



A tetraploid Narcissus has been reported by de Mol. 

 The diploid species has 14 chromosomes (7 pairs) while 

 two cultivated varieties were found to have 28 chromo- 

 somes. De Mol points out that until 1885 the small diploid 

 varieties were chiefly cultivated. Then the larger triploid 

 types appeared and finally about 1899 the first tetraploid 

 was obtained. 



The perennial teosinte of Mexico has twice as many 

 chromosomes as the annual teosinte, according to Long- 

 ley. The perennial Fig. 77a, has 40 chromosomes (n=20) 

 and the annual 20 chromosomes (n=10) Fig. 77c. Longley 

 crossed both of these with corn (maize), that has 20 

 chromosomes (n=10) Fig. 77b. The hybrid between the 

 annual teosinte and corn has 20 chromosomes. At the 

 maturation stages of the pollen mother cells there are 10 

 bivalents, and these divide and pass to the poles without 

 any lagging chromosomes. This means that the 10 chro- 

 mosomes that have come from the teosinte conjugate with 

 the 10 that have come from the corn. "When the perennial 

 teosinte is crossed to corn the hybrid has 30 chromo- 

 somes. At the ripening of the pollen mother cells of the 

 hybrid the chromosomes are found to be united, some in 

 threes, others in twos; the rest have no partners (Fig. 

 77ab). This leads to irregularities in the division that 

 follows (Fig. 77aV). 



In hermaphroditic or monoecious plants, where the 

 question of sex determination is not involved with differ- 

 ential sex-chromosomes, the tetraploid may be said to be 

 both balanced and stable. By balanced is meant that the 

 numerical relations of the genes is the same as that in the 

 diploid or normal type. By stable is meant that the 

 mechanism of maturation is such that the type, once 

 established, perpetuates itself. 3 



Tetraploids in mosses were produced as early as 1907 



3 Blakeslee used the terms differently. 



